The wide acceptance of unshielded twisted pair (UTP) systems for data and voice transmission is due to the large installed base of systems, their low cost and ease of new installation. Increased demands on networks using UTP systems, such as transmission rates, have forced the development of industry standards for higher system performance. Systems for simple telephone service and low speed network systems have become high speed data systems. As the speeds have increased, so too has system noise, in particular, near end crosstalk (NEXT).
For any data transmission event, the received signal will include the transmission signal, which is modified by various distortions. The various distortions are added by the transmission system, along with additional unwanted signals that are inserted somewhere between transmission and reception. The unwanted signals are referred to as noise. This noise is the major limiting factor in the current performance of communication systems. Problems that arise from noise are data errors, system malfunctions and loss of the actual wanted signals.
Noise may be divided into four categories, thermal noise, intermodular noise, impulse noise, and crosstalk. This invention is directed to crosstalk noise.
Crosstalk noise occurs when a signal from one source is coupled to another line. Crosstalk noise could also be classified as electromagnetic interference (EMI). EMI occurs through the radiation of electromagnetic energy. Electromagnetic energy waves can be derived by Maxwell's wave equations. These equations are basically defined using two components, electric and magnetic fields. In unbounded free space a sinusoidal disturbance propagates as a transverse electromagnetic wave (TEM wave). The electric field vectors are perpendicular to the magnetic field vectors which lie in a plane perpendicular to the direction of the wave. When one transmission pair (line A) is next to another pair (line B), crosstalk coupling can occurs. The fields are either capacitively (electric) or inductively (magnetic) coupled or both from line A to line B.
Crosstalk in a communication system can be minimized by decreasing the signal's power, increasing the distance between signal lines, providing shielding between signal lines, or twisting the wires of a pair. In a differential signaling communication system, plugs like RS-422A 25-pin, EIA T568B (AT&T) and ISDN 8-pin (ISO 8877) have predefined contact assignments. The predefined contact layouts eliminate all of the above crosstalk reduction methods, because restructuring is not an option. The ISO 8877 8-pin plug assignments and other similar plugs are not designed for high frequency use. These plugs produce crosstalk by the nature of their design. For example, a standard plug may have the following assignments for its eight pins: pin 1, power source; pin 2, power source; pin 3; transmit TA; pin 4, receive RA; pin 5, receive RB; pin 6, receive TB; pin 7, power sink; and pin 8, power sink.
Crosstalk in conventional plugs will increase as the speeds or system transmission frequencies increase. Crosstalk generated in the cable is minimized by the transposition (twisting) of the transmitting pair wires. However, as the signal travels through untwisted sections such as plugs and plug contacts, coupling occurs.
The transmitting signal could represent two signals in a differential balance signal system. In a differential balance transmission system, the signals traveling along a media are equal in amplitude, but opposite in phase. The phase difference of the two signals is .+-..eta. radian or voltage 1(E1)=-voltage 2(-E2) under ideal conditions. These signals at any instantaneous time couple electric and/or magnetic fields to adjacent lines which reduces their signal to noise ratio (S/N). The acceptable S/N ratio depends on the type or quality of service that is required by the system. To remove the noise components, a signal equal, but opposite, to the original signal is induced. According to Fourier's wave theory and Maxwells theory of electromagnetic fields, coupling the opposite phase, i.e. +.o slashed., of the transmitted signal to a previously coupled adjacent line signal, i.e. -.o slashed., the two signals cancel each other completely, thereby removing the noise from the adjacent line.